During glass formation from the melt, contaminants in the melt tend to form gas bubbles, also referred to in the art as “seeds.” Such seeds affect the performance and quality of the glass, and efforts are made to remove or “fine” them from the glass.
Seed formation is problematic for silicate glasses. In particular, aluminosilicate glasses and other silicate glasses that melt at high temperature are much more difficult to fine than other glasses. The high viscosities of such glasses slow the rate of bubble removal by via Stokes fining; i.e., allowing the bubbles to rise to the surface of the melt due to buoyancy.
Fining agents such as As2O3, Sb2O3, and halides have been used to remove bubbles from aluminosilicate glasses. These chemical fining packages work by releasing gas to existing bubbles, causing them to increase in size and rise more quickly to the top of the melt. However, these components are toxic, hazardous to handle, expensive, and undesirable for environmentally green products and processes. Sulfate fining agents have also been used in soft glasses. However, they contribute to sulfur emissions and actually exacerbate seed formation in aluminosilicate glasses.